Indoor air quality is a paramount issue in the heating, ventilation and air conditioning (HVAC) industry. Modern construction techniques greatly reduce the amount of outside air that is naturally introduced into structures through infiltration. The reduction of infiltration is a by-product of the quest to increase the energy efficiency in these structures. In recent years, great strides have been made in building techniques to provide airtight seals around doors, windows and other openings throughout these structures. All joints and cracks around the ceilings, walls and floors are carefully sealed during construction and all openings in these interior surfaces are made airtight. Air conditioning ceiling registers, the associated ductwork and the air handling systems are sealed to eliminate virtually all infiltration. The overall quality of structural sealing is measured using blower doors temporarily installed in a doorway to raise the air pressure inside the structure and determine the amount of air leaking from the structure. Infrared thermometers are used to identify and seal infiltration sources that change the air temperature inside the structure.
Increased structural energy efficiency combined with higher efficiency heating and cooling systems result in shorter run times and longer run intervals in the HVAC systems. This effect is causing increased air stagnation problems inside these modem structures. Carbon dioxide levels rise rapidly in smaller rooms with multiple occupants where air exchange times are short and infrequent. This problem is also exacerbated, during temperate weather periods, by personal security concerns associated with opening windows to provide natural outside air ventilation. The potential threat of airborne toxins that can be spread either by accident or by terrorist organizations has caused further concerns about the use of natural ventilation.
A wide array of synthetic building materials, such as carpeting, drapes, paint and vinyl flooring that naturally emit hazardous chemical vapors, are being used inside these structures. Combustible materials such as oil, gas, wood and tobacco products, building materials, cleaning and maintenance materials and pesticides provide additional sources of hazardous chemical vapors. This brew of chemicals, combined with the carbon dioxide naturally produced by the inhabitants, are causing respiratory problems. These problems seem to be more prevalent in higher humidity areas where higher levels of water vapor interact with the chemicals to produce other chemical compounds. The respiratory problems are particularly acute in children and the elderly. Standards established by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE Standard 62-1999, “Ventilation for Acceptable Indoor Air Quality”) require an average ventilation rate of 15 cubic feet per minute (CFM) of fresh air per person in structures. The average household size in the United States now stands at about 2.8 occupants per residence and this equates to an average of 42 CFM of outside air to be introduced on a continuous basis to every residence in the United States. This ventilation flow rate represents about three and one half percent of the total airflow produced by a 1200 CFM system fan, which equates to about three tons of cooling capacity. If this system runs intermittently for only one third of the time then the corresponding percentage increases to about ten and one half percent or 126 CFM. Few modern structures meet this standard.
Some notable efforts have been made in prior art to help alleviate these indoor air quality problems. A device (Rudd, U.S. Pat. No 6,431,268) was developed that monitors the time interval between operation of the HVAC system, or the operation of an outside air damper, and creates additional operating cycles of these devices to help alleviate problems associated with stagnant indoor air quality. Another approach (Riley et al., U.S. Pat. No. 6,467,696) uses continuously operating variable speed system fans that bring in outside air, mix the outside air with the indoor air and distribute the air throughout the structure. An earlier approach (Janu et al., U.S. Pat. No. 5,597,354) controls a single system fan and uses separate chambers for return air and outside air with associated dampers that control the airflow volumes in each chamber. A separate outside air damper is controlled to provide a variable intake of outside air. The invention supports a variety of pressure and contamination sensors located in both indoor and outside locations.
In response to the need to improve indoor air quality, some structures have been equipped with continuously open outside air ducts that include a manually adjustable damper to control how much outside air is introduced on a continuous basis while the system fan is operating. Such systems can cause dramatic reductions in the HVAC operating efficiency and in effect are similar to leaving a window or door partially open at all times. Other systems use an electrically controlled damper that opens when the system fan is operated and closes when it stops. Although this implementation does reduce air infiltration when the system fan is not running, these dampers typically do not provide an airtight seal when closed and some infiltration occurs when the dampers are closed. This infiltration problem is compounded by the buildup of static pressure inside the structure caused by introducing ventilation air and not providing a method to exhaust contaminated air. The increased pressure inside the structure above normal atmospheric pressure outside the structure also acts to limit the flow of ventilation air while the system fan is running. These systems introduce outside air even when the air is particularly hot, cold, humid or contaminated. The injection of extremely cold air during the heating season or extremely hot or humid air during the cooling season may result in significant decreases in energy efficiency. Injecting contaminated air reduces indoor air quality. Conversely, when the outside air is suitable for ventilation, these systems do not purposefully supplement the normal system fan operation to allow more ventilation. These periods typically occur in the spring and fall of the year when outdoor conditions often dictate the infrequent operation of an HVAC system.
Prior art has not considered sampling the condition of the outside air inlet stream as it enters the structure or the need to provide additional cycles of ventilation air whose quality is controlled to achieve proper ventilation. These prior art systems have not been widely accepted and are typically impractical for use in existing structures due to their cost and complexity. These systems are typically provided as separate devices that only address smaller aspects of total ventilation system needs. There are no standards available for installing and interfacing these separate devices and consequently there is wide variety of ventilation results. There is a need for fully integrated ventilation systems that address the total ventilation needs of a structure. These systems need to be simpler, lower in cost and more compact in size so they can be readily added to any structure. The limitations posed by the buildup of static pressure inside the structure have also been generally handled as a separate capability that is not fully integrated with the ventilation system. The availability of local air pollution data, including the potential release of airborne toxins has not been considered as a component of ventilation systems. The rapid communication of this alert situation, to all structures in an affected area, to curtail natural ventilation, has not been previously considered. The primary emphasis of prior art has been on limited capability systems that stir stagnant air inside the structure or monitor indoor sensors and supply enough fresh air to satisfy indoor air quality requirements. Accordingly, there is a need for a method and apparatus that provides a fully integrated system for sampling and controlling the outside air inlet stream as it enters the structure so that air quality, energy efficiency and personal safety are all attained in concert.